The prior art is replete with different types of electronic navigation systems. Some electronic navigation systems are handheld, and others are vehicle-based. A vehicle navigation system generally provides navigation instructions, location data, and map information to the vehicle operator. Some existing vehicle navigation systems attempt to optimize a route based upon different factors. Route calculation is typically performed by examining a number of possible paths, and selecting the “best” path according to a number of optimization rules. For instance, the shortest possible route may be chosen to minimize the distance traveled or high-speed roads may be chosen to minimize travel time. Some advanced navigation systems utilize real-time traffic congestion data in an attempt to guide the vehicle away from traffic jams. After the optimization criteria have been selected, automated vehicle route guidance is typically performed in a two-step process: (1) a proposed route is calculated from a starting position of the vehicle to the desired destination; and (2) guidance instructions are presented to the vehicle operator as the vehicle traverses the proposed route.
Many existing vehicle navigation systems will automatically calculate an updated route whenever the vehicle diverts from the previously calculated route. For example, after the system detects that the vehicle has turned at an intersection rather than proceed straight as originally designated, it will generate a new route to the destination, based upon the current location of the vehicle. Recalculation of an alternate route can take an appreciable amount of time (associated with the determination that the vehicle did not follow the original route, and with the actual computation of the new route), which can be inconvenient and confusing for the driver.
Existing vehicle navigation systems annunciate and/or display guidance instructions to the driver in advance of the next approaching intersection, transition, turnoff, or driving maneuver. For example, a system might display and audibly announce “Exit At Maynard Avenue” one or more times as the vehicle approaches that turnoff. However, the instruction “Exit At Maynard Avenue” in and of itself does not convey whether or not the driver “MAY exit,” or “SHOULD exit,” or “MUST exit” at Maynard Avenue. In this context, if Maynard Avenue is the last freeway exit for twenty miles, then the instruction ought to be treated as mandatory, important, or critical. On the other hand, if the driver can easily backtrack to Maynard Avenue by taking the immediately following freeway exit, then the instruction need not be treated as mandatory, important, or critical. Existing vehicle navigation systems do not alter or modulate the manner in which driving instructions are presented as a function of the importance of the driving maneuvers.